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THE ACIDIFICATION OF SCOTTISH LOCHS AND THE DERIVATION OF CRITICAL SULPHUR LOADS FROM PALAEOLIMNOLOGICAL DATA Richard W. Battarbee, Palaeoecology Research Unit, University College London, 26 Bedford Way, London WC1H OAP. INTRODUCTION A decade ago very little was known about acid lochs in Scotland. The implicit assumption was that they were largely free from pollution. In recent years the position has changed and we now have large bodies of data on the chemistry and biology of upland lochs (Harriman et al. 1986, Maitland et al. 1987, Battarbee et al. 1988) and clear evidence for the recent acidification of many sensitive sites (Flower and Battarbee 1983, Battarbee et al. 1985, Flower et al. 1987, Battarbee et al. 1988). Information on the acidification (pH) history of lochs has been obtained from the analysis of fossil diatom assemblages extracted from dated sediment cores (Battarbee 1984). Diatom-based pH reconstructions have now been completed for 20 Scottish lochs. This brief paper uses these data to describe the distribution of acidified lochs in Scotland in relation to the pattern of total sulphur (S) deposition and shows how such data can be used to define critical S loads for individual sites. The distribution of sites studied is shown in Figure 1. Almost all sites are sensitive to acidification with values for calcium (Ca") less than 100 )leq I" (Table 1). Only Loch Urr in Galloway has a higher value (170 Ileq 1"). This site was selected as a high pH, high calcium comparison to other, more acidic sites in Galloway. Sites throughout the rest of Scotland were selected to represent most of the sensitive regions of the country as well as to form a series along a gradient of decreasing total S deposition from south to the north-west. Some sites for which results are not yet available, mainly in the north-west of Scotland, are also shown (Figure 1). REGIONAL PATTERNS The diatom-based pH reconstruction for each site is shown in Table 2 and the following account considers these data region be region. 1. Galloway In Galloway the most acidified lakes occur on the Loch Doon and Cairnsmore of Fleet granites. Three sites with moorland catchments situated entirely on the Loch Doon granite: Loch Enoch, Loch Valley and the Round Loch of Glenhead, have become gradually more acid since the mid-nineteenth century. Loch Enoch lost its fish population in the 1950s and fish populations in Loch Valley and the Round Loch of Glenhead are poor. Loch Dee situated partly on the granite and at a lower elevation has been less strongly acidified and acidification began somewhat later, towards the end of the nineteenth century and well before conifer planting in the 19705. 1 The response of the three main lakes on the Caimsmore granite (Loch Grannoch, Loch Fleet, Loch Skerrow) to acid deposition has been complicated by the additional effects of afforestation (including deep ploughing), hydrological disturbance and geological variability. Rapid acidification of Loch Grannoch began in the early twentieth century prior to afforestation, but drainage and conifer planting in the 1960s and 1970s has caused soil erosion and may have exacerbated acidification. At Loch Fleet, unusually high groundwater alkalinity caused by secondary calcite veining in the granite (Cook et al. 1987) was probably responsible for this site maintaining a relatively high pH until after afforestation. Afforestation was accompanied by substantial soil inwash to the lake and the rapid post-1970 acidification may have been caused more by the combined effects of inwash and afforestation than by acid deposition. The situation at Loch Skerrow is more complex. Only slight acidification has occurred at this site. The very acid drainage from the afforested granitic parts of the catchment to the west (pH 4.0-4.5) is continually countered by much more alkaline inflows from the east (pH 6.5) where the catchment is underlain by Silurian sedimentary strata. The main high-pH inflow is an artificial leat associated with the water supply to the former Loch Skerrow railway halt. All the sites on the granite are senSItIve to acidification (Ca++ 20-70 lleq n. As a comparison a core from Loch Urr, a nearby site uninfluenced by granitic geology with Ca++ of 166 lleq I" and pH 6.7, was analysed. Despite a clear record of trace metal and carbonaceous particle contamination, no acidification has occurred. 2. Arran Surface waters on granitic rocks in the north of Arran are acidified. Cores from Loch Tanna show that the lake was naturally very acid prior to 1800 with pH between 5.0 and 5.5. However, diatom changes from 1850 onwards indicate a progressive acidification to the present day with a decline of about 0.5 pH. 3. Loch Ard forest and the Trossachs Two sites in this region have been cored to assess the impact of afforestation. Loch Chon and Loch Tinker are situated on the same Upper Dalradian quartzose-schist lithology. The catchment of Loch Chon is extensively afforested whereas Loch Tinker has an open moorland catchment. The analyses show that Loch Tinker has been slightly acidified since 1850 and that Loch Chon has been strongly acidified only after the planting of conifers (post-1970). In this area where catchments have a greater neutralising capacity than the Galloway granites (Loch Tinker and Loch Chon have Ca++ of 70-80 lleq 1"), it is apparent that afforestation has helped to promote acidification, probably because of the increased scavenging of dry and occult deposition by the forest canopy. 2 4. Rannoch Moor Surface waters on the granite of Rannoch Moor are very sensitive to acidification (Ca++ 40 Ileq 1'1). A core from Loch Laidon, an important fishing loch and part of a National Nature Reserve, shows an acidification of ahout 0.5 pH from ahout pH 5.8 to pH 5.3 between 1850 and the present. 5. Cairngorms and Lochnagar High corrie lochs on the Lochnagar granite (Lochnagar, Dubh Loch and Loch nan Eun) have been acidified by between 0.5 and 1 pH unit from ahout 1850 to the present day. All sites are extremely remote and at high elevation (>500 m) yet are strongly contaminated by trace metals and carbonaceous particles. Lochan Uaine in the main Cairngorm region is similarly contaminated but is less sensitive to acidification (Ca++ 60 Ileq I") and no pH decrease has occurred. 6. Strontian and Morvern Data are available for Lochan Dubh and Loch Doilet in this region of relatively low acid deposition. Lochan Dubh has a moorland catchment whilst the catchment of Loch Doilet has been extensively afforested. Both are very sensitive sites and have been acidified by about 0.3 pH unit. The apparent lack of a "forest effect" at Loch Doilet, compared to the effect at Loch Chon (Table 2), suggests that conifer growth by itself does not cause acidification. Loch Uisge. a site somewhat to the south of these sites is less sensitive (Ca++ 66 Ileq I") and no pH change has occurred. 7. The North and North-west The North and North-west highlands and islands of Scotland are areas of relatively low acid deposition. Work is in progress on sediments from a number of sites. No clear-water (TOC <5 mg I") lakes with pH <5.5 are known in this region yet many are sensitive to acidification with Ca++ <50 Ileq I"). So far only data from Loch Coire nan Art, a clear water loch with pH of 6.3 are available. Despite its sensitivity (similar to the Round Loch of Glenhead), no pH change has occurred at this site, and carbonaceous particle concentrations are very low. 3 COMPARISONS WITH TOTAL S DEPOSITION AND THE DERIVATION OF CRITICAL LOADS Figure 2 shows the pattern of pH change of lochs in Scotland compated with contours of total S deposition. The sites for which no pH change has occurred ate indicated by open circles and include Loch Coire nan Arr, Loch Uisge, Lochan Uaine, and Loch Urr. The location of these sites in relation to the pattern of S deposition illustrates the cleat relationship between sensitivity and deposition. Only .in the north-west of Scotland, where total S deposition is approximately 0.5 g Srn" yr1 or less, ate very sensitive sites (Ca++ <50 lleq 1'1) not acidified. In Galloway, where the highest total S deposition occurs, sensitive sites ate strongly acidified and only sites with Ca++ >100 lleq 1,1 ate not acidified. The only significant qualification to this pattern is in the case of afforested catchments where sites such as Loch Chon ate more strongly acidified than would be predicted from this relationship. In Figure 3 total S deposition at each site is plotted against calcium, as an indicator of sensitivity and the diatom-based acidification stams of each site is shown. Although there ate relatively few data points for non-acidified lochs it is possible to indicate an approximate calcium to deposited sulphur ratio which differentiates acidified from non-acidified waters. This line or ratio then .allows the critical S load for any site to be calculated. For example the critical load for Loch Enoch, with a Ca++ value of about 20 lleq 1'1, is approximately 0.3 g Srn" y.l. To return this site to a non-acidified status, assuming base cation values have and will remain approximately constant as deposition is reduced (cf. Henriksen 1984), requires a reduction in S deposition from about 1.25 to 0.3 g m,l y.l, or approximately 75%. On the other hand for Loch Tinker, a less acidified, less sensitive site although in an atea of somewhat higher S deposition (1,4 gm" y'l) only a very slight reduction should be sufficient for a return to its non-acidified status.

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